The present invention relates to a ball bearing for use in a hard disc apparatus for supporting, for example, a rotary member of a spindle motor or supporting a rotary member in a pivot section of a magnetic head in the apparatus.
Conventionally, the hard disc apparatus is provided with the spindle motor for high speed rotation of a magnetic disc as a magnetic recording medium. The apparatus employs a ball bearing for supporting a hub which is the rotary member of the spindle motor. Unfortunately, if a natural frequency of the ball bearing matches a natural frequency of a component other than the ball bearing, there may occur resonance therebetween which will cause abnormal vibrations interfering with writing data to or reading data from the magnetic disc. Accordingly, it is a general practice to change materials for or shapes of the individual components for differentiating the natural frequency of the ball bearing from those of the other components, thereby avoiding the occurrence of resonance therebetween.
However, the natural frequencies of the ball bearings vary greatly from one bearing product to another and so are the mean values thereof. Therefore, it is actually difficult to differentiate the natural frequencies of the other components from those of all the conceivable ball bearings. Thus, there is a problem of inability to effectively prevent the occurrence of resonance. Such a problem is encountered not only by the ball bearing of the spindle motor but also by the ball bearing disposed in the pivot section of the magnetic head assembly.
A ball bearing according to the invention comprises: an outer ring having a raceway; an inner ring disposed in the outer ring in concentric relation therewith and having a raceway opposing the raceway of the outer ring; and a plurality of balls interposed between the raceways of the outer and inner rings, wherein within bounds on both sides of an axial center of each of the raceways of the outer and inner rings, a curvature radius of a raceway portion in a dimensional range covering at least 45% of a raceway width with respect to the axial center is within the range of between xc2x11 xcexcm to/from a mean curvature radius of the raceway in the dimensional range.
According to the ball bearing of such a configuration, the natural frequencies and the mean values thereof are prevented from greatly varying from one product to another. Specifically, the present inventors have made intensive study on the causes of the product-by-product great variations in the natural frequency of the ball bearing and in the mean value of the natural frequencies thereof, to find that minor shape irregularities produced on the raceways of the inner and outer rings substantially affect such variations. The inventors discovered that if the curvature radius of the raceway portion at least in the aforesaid dimensional range is limited within the range of between xc2x11 xcexcm to/from the mean curvature radius thereof, the ball bearings are prevented from suffering the product-by-product great variations in the natural frequency and the mean value thereof. Thus, the inventors have achieved the invention based on this discovery.
According to the inventive ball bearing, the product-by-product great variations in the natural frequency and the mean value thereof are prevented and hence, it is easy to estimate the natural frequency and the mean value thereof. Thus, it becomes easier to design and fabricate the apparatus with the aim of differentiating the natural frequency of the bearing assembled in the apparatus from the natural frequencies of the other components of the apparatus. As a result, the occurrence of resonance between the bearing assembled in the apparatus and the other components thereof is effectively prevented.